Coupled carbon and nitrogen losses in response to seven years of chronic warming in subarctic soils

Marañón-Jiménez S., Peñuelas J., Richter A., Sigurdsson B.D., Fuchslueger L., Leblans N.I.W., Janssens I.A. (2019) Coupled carbon and nitrogen losses in response to seven years of chronic warming in subarctic soils. Soil Biology and Biochemistry. 134: 152-161.
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Doi: 10.1016/j.soilbio.2019.03.028

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Increasing temperatures may alter the stoichiometric demands of soil microbes and impair their capacity to stabilize carbon (C) and retain nitrogen (N), with critical consequences for the soil C and N storage at high latitude soils. Geothermally active areas in Iceland provided wide, continuous and stable gradients of soil temperatures to test this hypothesis. In order to characterize the stoichiometric demands of microbes from these subarctic soils, we incubated soils from ambient temperatures after the factorial addition of C, N and P substrates separately and in combination. In a second experiment, soils that had been exposed to different in situ warming intensities (+0, +0.5, +1.8, +3.4, +8.7, +15.9 °C above ambient) for seven years were incubated after the combined addition of C, N and P to evaluate the capacity of soil microbes to store and immobilize C and N at the different warming scenarios. The seven years of chronic soil warming triggered large and proportional soil C and N losses (4.1 ± 0.5% °C −1 of the stocks in unwarmed soils) from the upper 10 cm of soil, with a predominant depletion of the physically accessible organic substrates that were weakly sorbed in soil minerals up to 8.7 °C warming. Soil microbes met the increasing respiratory demands under conditions of low C accessibility at the expenses of a reduction of the standing biomass in warmer soils. This together with the strict microbial C:N stoichiometric demands also constrained their capacity of N retention, and increased the vulnerability of soil to N losses. Our findings suggest a strong control of microbial physiology and C:N stoichiometric needs on the retention of soil N and on the resilience of soil C stocks from high-latitudes to warming, particularly during periods of vegetation dormancy and low C inputs. © 2019 Elsevier Ltd

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Isotopic evidence for oligotrophication of terrestrial ecosystems

Craine J.M., Elmore A.J., Wang L., Aranibar J., Bauters M., Boeckx P., Crowley B.E., Dawes M.A., Delzon S., Fajardo A., Fang Y., Fujiyoshi L., Gray A., Guerrieri R., Gundale M.J., Hawke D.J., Hietz P., Jonard M., Kearsley E., Kenzo T., Makarov M., Marañón-Jiménez S., McGlynn T.P., McNeil B.E., Mosher S.G., Nelson D.M., Peri P.L., Roggy J.C., Sanders-DeMott R., Song M., Szpak P., Templer P.H., Van der Colff D., Werner C., Xu X., Yang Y., Yu G., Zmudczyńska-Skarbek K. (2018) Isotopic evidence for oligotrophication of terrestrial ecosystems. Nature ecology & evolution. 2: 1735-1744.
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Doi: 10.1038/s41559-018-0694-0

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Human societies depend on an Earth system that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations and isotope ratios (δ15N) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar δ15N declined by 0.6-1.6‰. Examining patterns across different climate spaces, foliar δ15N declined across the entire range of mean annual temperature and mean annual precipitation tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ15N of tree rings and leaves from herbarium samples over the past 75-150 years. These patterns are consistent with the proposed consequences of elevated atmospheric carbon dioxide and longer growing seasons. These declines will limit future terrestrial carbon uptake and increase nutritional stress for herbivores.

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Ancillary vegetation measurements at ICOS ecosystem stations

Gielen B., Acosta M., Altimir N., Buchmann N., Cescatti A., Ceschia E., Fleck S., Hörtnagl L., Klumpp K., Kolari P., Lohila A., Loustau D., Marańon-Jimenez S., Manise T., Matteucci G., Merbold L., Metzger C., Moureaux C., Montagnani L., Nilsson M.B., Osborne B., Papale D., Pavelka M., Saunders M., Simioni G., Soudani K., Sonnentag O., Tallec T., Tuittila E.-S., Peichl M., Pokorny R., Vincke C., Wohlfahrt G. (2018) Ancillary vegetation measurements at ICOS ecosystem stations. International Agrophysics. 32: 645-664.
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Doi: 10.1515/intag-2017-0048

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The Integrated Carbon Observation System is a Pan-European distributed research infrastructure that has as its main goal to monitor the greenhouse gas balance of Europe. The ecosystem component of Integrated Carbon Observation System consists of a multitude of stations where the net greenhouse gas exchange is monitored continuously by eddy covariance measurements while, in addition many other measurements are carried out that are a key to an understanding of the greenhouse gas balance. Amongst them are the continuous meteorological measurements and a set of non-continuous measurements related to vegetation. The latter include Green Area Index, aboveground biomass and litter biomass. The standardized methodology that is used at the Integrated Carbon Observation System ecosystem stations to monitor these vegetation related variables differs between the ecosystem types that are represented within the network, whereby in this paper we focus on forests, grasslands, croplands and mires. For each of the variables and ecosystems a spatial and temporal sampling design was developed so that the variables can be monitored in a consistent way within the ICOS network. The standardisation of the methodology to collect Green Area Index, above ground biomass and litter biomass and the methods to evaluate the quality of the collected data ensures that all stations within the ICOS ecosystem network produce data sets with small and similar errors, which allows for inter-comparison comparisons across the Integrated Carbon Observation System ecosystem network. © 2018 Bert Gielen et al., published by Sciendo 2018.

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Sampling and collecting foliage elements for the determination of the foliar nutrients in ICOS ecosystem stations

Loustau D., Altimir N., Barbaste M., Gielen B., Jiménez S.M., Klumpp K., Linder S., Matteucci G., Merbold L., De Beek M.O., Soulé P., Thimonier A., Vincke C., Waldner P., Marañon- Jimenez S. (2018) Sampling and collecting foliage elements for the determination of the foliar nutrients in ICOS ecosystem stations. International Agrophysics. 32: 665-676.
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Doi: 10.1515/intag-2017-0038

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The nutritional status of plant canopies in terms of nutrients (C, N, P, K, Ca, Mg, Mn, Fe, Cu, Zn) exerts a strong influence on the carbon cycle and energy balance of terrestrial ecosystems. Therefore, in order to account for the spatial and temporal variations in nutritional status of the plant species composing the canopy, we detail the methodology applied to achieve consistent time-series of leaf mass to area ratio and nutrient content of the foliage within the footprint of the Integrated Carbon Observation System Ecosystem stations. The guidelines and defi-nitions apply to most terrestrial ecosystems. © 2018 Denis Loustau et al., published by Sciendo 2018.

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Geothermally warmed soils reveal persistent increases in the respiratory costs of soil microbes contributing to substantial C losses

Marañón-Jiménez S., Soong J.L., Leblans N.I.W., Sigurdsson B.D., Peñuelas J., Richter A., Asensio D., Fransen E., Janssens I.A. (2018) Geothermally warmed soils reveal persistent increases in the respiratory costs of soil microbes contributing to substantial C losses. Biogeochemistry. : 1-16.
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Doi: 10.1007/s10533-018-0443-0

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Increasing temperatures can accelerate soil organic matter decomposition and release large amounts of CO2 to the atmosphere, potentially inducing positive warming feedbacks. Alterations to the temperature sensitivity and physiological functioning of soil microorganisms may play a key role in these carbon (C) losses. Geothermally active areas in Iceland provide stable and continuous soil temperature gradients to test this hypothesis, encompassing the full range of warming scenarios projected by the Intergovernmental Panel on Climate Change for the northern region. We took soils from these geothermal sites 7 years after the onset of warming and incubated them at varying temperatures and substrate availability conditions to detect persistent alterations of microbial physiology to long-term warming. Seven years of continuous warming ranging from 1.8 to 15.9 °C triggered a 8.6–58.0% decrease on the C concentrations in the topsoil (0–10 cm) of these sub-arctic silt-loam Andosols. The sensitivity of microbial respiration to temperature (Q10) was not altered. However, soil microbes showed a persistent increase in their microbial metabolic quotients (microbial respiration per unit of microbial biomass) and a subsequent diminished C retention in biomass. After an initial depletion of labile soil C upon soil warming, increasing energy costs of metabolic maintenance and resource acquisition led to a weaker capacity of C stabilization in the microbial biomass of warmer soils. This mechanism contributes to our understanding of the acclimated response of soil respiration to in situ soil warming at the ecosystem level, despite a lack of acclimation at the physiological level. Persistent increases in the respiratory costs of soil microbes in response to warming constitute a fundamental process that should be incorporated into climate change-C cycling models. © 2018 Springer International Publishing AG, part of Springer Nature

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Importance of reporting ancillary site characteristics, and management and disturbance information at ICOS stations

Saunders M., Dengel S., Kolari P., Moureaux C., Montagnani L., Ceschia E., Altimir N., López-Ballesteros A., Marańon-Jimenez S., Acosta M., Klumpp K., Gielen B., De Beeck M.O., Hörtnagl L., Merbold L., Osborne B., Grünwald T., Arrouays D., Boukir H., Saby N., Nicolini G., Papale D., Jones M. (2018) Importance of reporting ancillary site characteristics, and management and disturbance information at ICOS stations. International Agrophysics. 32: 457-469.
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Doi: 10.1515/intag-2017-0040

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There are many factors that influence ecosystem scale carbon, nitrogen and greenhouse gas dynamics, including the inherent heterogeneity of soils and vegetation, anthropogenic management interventions, and biotic and abiotic disturbance events. It is important therefore, to document the characteristics of the soils and vegetation and to accurately report all management activities, and disturbance events to aid the interpretation of collected data, and to determine whether the ecosystem either amplifies or mitigates climate change. This paper outlines the importance of assessing both the spatial and temporal variability of soils and vegetation and to report all management events, the import or export of C or N from the ecosystem, and the occurrence of biotic/abiotic disturbances at ecosystem stations of the Integrated Carbon Observation System, a pan-European research infrastructure. © 2018 Matthew Saunders et al., published by Sciendo 2018.

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Soil microbial CNP and respiration responses to organic matter and nutrient additions: Evidence from a tropical soil incubation

Soong J.L., Marañon-Jimenez S., Cotrufo M.F., Boeckx P., Bodé S., Guenet B., Peñuelas J., Richter A., Stahl C., Verbruggen E., Janssens I.A. (2018) Soil microbial CNP and respiration responses to organic matter and nutrient additions: Evidence from a tropical soil incubation. Soil Biology and Biochemistry. 122: 141-149.
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Doi: 10.1016/j.soilbio.2018.04.011

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Soil nutrient availability has a strong influence on the fate of soil carbon (C) during microbial decomposition, contributing to Earth's C balance. While nutrient availability itself can impact microbial physiology and C partitioning between biomass and respiration during soil organic matter decomposition, the availability of labile C inputs may mediate the response of microorganisms to nutrient additions. As soil organic matter is decomposed, microorganisms retain or release C, nitrogen (N) or phosphorus (P) to maintain a stoichiometric balance. Although the concept of a microbial stoichiometric homeostasis has previously been proposed, microbial biomass CNP ratios are not static, and this may have very relevant implications for microbial physiological activities. Here, we tested the hypothesis that N, P and potassium (K) nutrient additions impact C cycling in a tropical soil due to microbial stoichiometric constraints to growth and respiration, and that the availability of energy-rich labile organic matter in the soil (i.e. leaf litter) mediates the response to nutrient addition. We incubated tropical soil from French Guiana with a 13C labeled leaf litter addition and with mineral nutrient additions of +K, +N, +NK, +PK and +NPK for 30 days. We found that litter additions led to a ten-fold increase in microbial respiration and a doubling of microbial biomass C, along with greater microbial N and P content. We found some evidence that P additions increased soil CO2 fluxes. Additionally, we found microbial biomass CP and NP ratios varied more widely than CN in response to nutrient and organic matter additions, with important implications for the role of microorganisms in C cycling. The addition of litter did not prime soil organic matter decomposition, except in combination with +NK fertilization, indicating possible P-mining of soil organic matter in this P-poor tropical soil. Together, these results point toward an ultimate labile organic substrate limitation of soil microorganisms in this tropical soil, but also indicate a complex interaction between C, N, P and K availability. This highlights the difference between microbial C cycling responses to N, P, or K additions in the tropics and explains why coupled C, N and P cycle modeling efforts cannot rely on strict microbial stoichiometric homeostasis as an underlying assumption. © 2018

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Impacts of global change on Mediterranean forests and their services

Peñuelas, J., Sardans, J., Filella, I., Estiarte, M., Llusià, J., Ogaya, R., Carnicer, J., Bartrons, M., Rivas-Ubach, A., Grau, O., Peguero, G., Margalef, O., Pla-Rabés, S., Stefanescu, C., Asensio, D., Preece, C., Liu, L., Verger, A., Barbeta, A., Achotegui-Castells, A., Gargallo-Garriga, A., Sperlich, D., Farré-Armengol, G., Fernández-Martínez, M., Liu, D., Zhang, C., Urbina, I., Camino-Serrano, M., Vives-Ingla, M., Stocker, B.D., Balzarolo, M., Guerrieri, R., Peaucelle, M., Marañón-Jiménez, S., Bórnez-Mejías, K., Mu, Z., Descals, A., Castellanos, A., Terradas, J. (2017) Impacts of global change on Mediterranean forests and their services. Forests. 8: 0-0.
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Doi: 10.3390/f8120463

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